1. After Wegner has published his book on the origin of continents and oceans in Scientists started to have interest in studying Wegner’s hypothesis. Later, the new data came from the least-known part of earth- The Ocean floor.

3. Exploring the ocean floor
In early 1900s, new technology helped the scientists to make it easy to discover the sea floor. It is SONAR ( sound navigation and ranging). T is a system that uses sound waves to calculate the distance to an object.
Sound waves off under-water objects and then records the echoes of these sound waves. The time it takes for the echo to arrive indicates the distance to the object.

4. The deeper the water, the longer it takes the sound waves to return back to he ship.

5. The process of sea-floor spreading
Harry Hess believed that the mid-ocean ridges and deep-ocean trenches might help to explain how ocean floor was formed.
His hypothesis was ‘ in the process of sea-floor spreading, new ocean floor forms along Earth’s mid-ocean ridges, slowly moves outward across ocean basins, and finally sinks back into the mantle beneath deep-ocean trenches.’
New oceanic lithosphere is formed, and the ocean floor gets wider. Example: The Atlantic ocean would have been like the Red Sea.

6. Eruption along Mid-Ocean Ridges
Scientists found evidence that is formed as a result of volcanic activity;
1.Fractures along central valley of a mid-ocean ridge fill with magma that wells up from the hot mantle below. Gradually magma cools produce new slivers ( strips of loose pieces) of ocean floor. Spreading and upwelling ( flowing upward) of magma continuously adds new ocean floor.

7. 2. The process can also begin on land when a rift valley forms and splits a continental landmass. Over a long period of time, the rift valley widens to form a new oceanic basin like the Red Sea.

8. Subduction at Deep-ocean Trenches
Erath’s total surface area remains the same, although new ocean floor is constantly being added at the mid-ocean ridges. This happens by Subduction ; it is returning the older portions of the ocean floor back to the mantle as it sinks beneath a deep ocean trench.
The area where subduction occurs are called subduction zones.

9. Evidence for sea floor spreading
Evidences that support Hess’s hypothesis;
Magnetic strips in ocean-floor rocks.
Earth has its magnetic field. This field could reverse its polarity ( the north magnetic pole becomes south pole). Earth is said to have a reverse polarity. Normal polarity means when Earth magnetic field lines up in the same direction as present magnetic field.

10. When some rocks form, they acquire the polarity of Erath’s magnetic field at that time. They posses Paleomagnetism.
Igneous rocks contain magnetite, iron-rich mineral. As the rock cools and hardens, the iron rich mineral grains become magnetized in the same direction of as the existing magnetic field.
When the rock has been formed, its polarity remains frozen unless it is reheated above certain temperature. The rock’s paleomagnetism does not change.

11. Using magnetometers to study the paleomagnetism of Basalt rock that makes up the ocean floor. The data showed alternating strips of normal and reverse polarity of magnetized rock. Scientists inferred that a new basalt rock forms along the mid-ocean ridges became magnetized according to the Earth’s polarity of that time.

13. 2. Earthquake patterns The depth of certain earthquakes
Scientists, Kiyoo Wadati and Hugo Benioff, found patterns between the depth of earthquakes in relation to their distance from deep-ocean trenches
Shallow-focus earthquakes occur in and around a trenches
Intermediate-focus and deep-focus earthquakes occur in a belt 50 kilometers thick.
The pattern was what scientists expected would result from subduction of the ocean floor. At subduction zones, the ocean floor returns to the mantle (destroying crust.

15. 3. The age of the ocean floor
Drilling into sediments on the ocean floor and the crust below it
Data confirmed:
Ocean floor is youngest along the central valley of the mid-ocean ridge
Ocean floor is the oldest in subduction zones or near the edges of continents far from the ridge.
Ocean floor is not older than 180 million years old.